1. Field of the Invention
The present invention relates to a nonlinear transmission line waveform generator for use as a harmonic generator and for generating relatively short duration pulses, for example, in the picosecond range, and more particularly to a nonlinear transmission line waveform generator for producing relatively short duration narrow pulses which do not experience the problems associated with nonlinear transmission lines which include step recovery diodes, tunnel diodes or varactor diodes.
2. Description of the Prior Art
Nonlinear transmission lines are generally known in the art. Examples of such nonlinear transmission lines are disclosed in: xe2x80x9cGaAs Nonlinear Transmission Lines for Picosecond Pulse Generation and Millimeter Wave Samplingxe2x80x9d, by Rodwell, et al., IEEE Transactions on Microwave Theory and Techniques, vol. 39, no. 7, July 1991, pages 1194-1204; U.S. Pat. Nos. 5,352,994; 5,352,627; 5,023,574; 5,256,996; 5,789,994; 5,014,018; and 5,157,361, all hereby incorporated by reference. Such nonlinear transmission lines are used in various applications, such as harmonic generators and generation of relatively short duration narrow pulses in the hundreds of picosecond range which provide increased bandwidth in certain applications, such as ultra wideband radio applications, also known as impulse radios. In particular, ultra wideband radios are generally known in the art. Examples of such ultra wideband radios are manufactured by Time Domain Corporation and Paulson Communication Corporation. Such ultra wideband radios utilize relatively short-duration pulses, for example, pulse widths in the hundreds of picoseconds. These short-duration pulses are pulse-position modulated, to increase the bandwidth of such radios up to, for example, 5 GHz. Unfortunately, there is an ever increasing demand to further increase the bandwidth of such impulse radios as well as other microwave devices. The bandwidth of such ultra-wideband radios is limited by the current technology in which the shortest duration pulses which can be generated are in the hundreds of picoseconds. Thus, there is a need to provide a system for generating pulses having pulse widths less than the hundreds of picoseconds in duration.
Moreover, some known methods for fabricating nonlinear transmission lines are complicated by the need for step recovery diodes, tunnel diodes or varactor diodes. More particularly, standard transmission lines are known to be modeled as a distributed network of series inductances L and shunt capacitances C. In nonlinear transmission lines, the shunt capacitances C are replaced with varactor diodes, step recovery diodes or tunnel diodes, all of which are known to cause several problems. First, such devices complicate monolithic fabrication. Also, such devices are known to become unstable and oscillate. Thus, a need has developed for simpler, less complicated devices that are also more stable than known nonlinear transmission lines.
Briefly, the present invention relates to a nonlinear transmission waveform generator for line for generating a comb of frequencies and relatively short duration pulses, for example, in the range of picoseconds and tens of picoseconds, that are adapted to being utilized with ultra-wideband radios in order to improve the bandwidth of such radios by an order of magnitude, for example, up to tens and even hundreds of GHz. In particular, the nonlinear transmission line waveform generator in accordance with the present invention consists of a microstrip or coplanar waveguide line. In accordance with an important aspect of the invention, the C/V characteristic of the nonlinear transmission line is matched to the frequency and amplitude of the input sinusoid. By matching the C/V characteristics of the nonlinear transmission line to the input sinusoid, the output of the nonlinear transmission line produces a comb of frequencies that are multiples of the input sinusoid frequency, making it suitable as a harmonic generator. The nonlinear transmission line can also be used to generate relatively short-duration pulses by employing a shorting stub at the output. The shorting stub causes the waveform to be reflected 180xc2x0 out of phase, so that it cancels the trailing edge of the original output from the nonlinear transmission lines to form a short duration pulse in the picosecond range. The length of the stub determines the width of the resultant output pulse.